Pineal organs in deep demersal fish

We studied ten species of demersal fish from depths of 1500-4800 m, i.e. regions of the abyss outside the reach of sunlight. A pineal window in the skin and/or the skull, often found in mesopelagic fish, was never observed in demersal specimens. Nine species had a well-developed pineal organ, with light- and electron-microscopic features, well known in other teleosts living in surface waters, including photoreceptor cells with inner and outer segments, synaptic ribbons, neuronal perikarya, and (radial) glial cells. One species ( Bathypterois dubius) showed signs of regression; it also had reduced eyes. We observed considerable morphological variation in location, size, microscopic structure and ultrastructural organisation, including the frequency of photoreceptor cells, size of outer segments and the number of myelinated and unmyelinated axons. No systematic trend in the sense of an increase of sensitivity with greater depths was observed. Melatonin contents varied between 4 pg and 92 pg per pineal in the grenadier Coryphaenoides ( Nematonurus) armatus and between 2 pg and 70 pg per pineal in the eel Synaphobranchus kaupi. Differences between day and night values and between autumn and spring suggest that pineal melatonin acts as neurochemical signal mediating rhythmic processes and behaviour. The role of an alternative non-solar zeitgeber in the demersal environment is discussed.